1纳米相当于1根头发丝直径的六万分之一.则利用科学记数法来表示,头发丝的半径是[ ]
A.6万纳米
B.6×104纳米
C.3×10﹣6米
D.3×10﹣5米
答案:D
Mary couldn’t have received my letter; otherwise she______before now.
A.would have replied B.has replied C.must have replied D.had replied
不会造成炉门跑烟冒火的是()
A.炉门损坏
B.集气管压力小
C.上升管、桥管或集气管堵